Arachnologische Mitteilungen 56

4 S. Toft argues that this place houses a large horticultural centre that is a target of truck traffic from all of Europe, including not least the Netherlands. Interestingly, the early Swedish records of D. ramosus were from the Helsingborg and the Malmö/ Lund area ( Jonsson 2013, Artportalen 2018), i.e. the part of Sweden adjacent to Copenhagen. Thus, the species probab- ly immigrated to Sweden via Denmark. No highways go up along the west coast of Jutland, but this area attracts high numbers of German tourists during the summer and early au- tumn.Thus, the areas in which D. ramosus became established very early are those which have relatively high traffic rates ori- ginating from south of Denmark. In contrast, central Jutland is characterized by east–west traffic and was colonized some years later.The first observation from the island of Bornholm, an isolated island in the Baltic Sea between Poland and Swe- den, came in 2017. The late arrival here was expected due to limited traffic connections (ferries from Copenhagen, Ger- many and Sweden). Considering the Swedish distribution of D. ramosus in 2017 (Artportalen 2018), the traffic connec- tions to Bornholm, and the absence of the species in eastern Germany (Arachnologische Gesellschaft 2018), immigration from the north via Sweden may be the most probable route. Comparison of Fig. 1A and 1B shows that during the years 2010 to 2017, D. ramosus in Jutland has filled out much of the space between the locations initially colonized. In 2017 the species was missed in very few of the urban counts and only from one location in which it had been observed in previous years. Fig. 3 witnesses a dramatic increase in the number of new localities reported in precisely this period.Though trans- portation by cars cannot be excluded as partial explanation for the short-distance dispersal, self-accomplished dispersal may also be partly involved in the local filling of available habitat space. The species’ preference for shrubby habitat (Noordijk et al. 2007, own observations) may facilitate corridor sprea- ding e.g. along hedges. Unfortunately, no direct observations exist to evaluate the relative importance of dispersal mecha- nisms. The Danish distribution of O. spinosus (Fig. 2) differs from that of D. ramosus in that it is completely missing in wes- tern Jutland, and that it is found considerably more often on the islands (Funen, Zealand, Lolland, Møn) than in Jutland. Apart from showing much slower rates of expansion, this may indicate that the routes of immigration are partly different. The occurrences in eastern Jutland and Funen may have follo- wed the same route as D. ramosus , i.e. via highway traffic from Germany.The relatively heavy concentration of the species on south Zealand, Lolland and Møn indicates another immig- ration route via the ferry from Germany to Rødby (southern coast of Lolland). The immigrants here may have originated from the population that have established in eastern Germa- ny. The possibility of this immigration route is documented by the extreme number of rare southern species (insects, mil- lipedes, centipedes, harvestmen) that have been found at an abandoned railway area near the ferry harbour (Enghoff et al. 2011). Wind dispersal has been proposed as a possible mecha- nism of dispersal for the expanding harvestmen (Noordijk et al. 2007). There are neither direct nor indirect evidence for this, however. Firstly, I am unaware of any observations of “flying” harvestmen; in contrast to spiders and mites, harvest- men seem never to have been recorded in samples of aerial plankton whether recorded from planes (Glick 1939), high masts (Freeman 1946) or from boats far at sea (Hardy & Cheng 1986). Secondly, if the air was a main dispersal medi- um, the direction of spreading is expected to follow the main wind direction. The direction of expansion of most harvest- men has been towards the north, while prevailing winds are westerly ( http://www.dmi.dk ) . Development of the reports for each of the two species to Naturbasen (2018) (Fig. 3) indicate that the largest increase in the number of locations from which they were reported came in the latter half of the period, after a relatively slow increase in the first half. This picture is consistent with the hypothesis that the early finds were due to independent im- migrations from abroad, while the later “filling up” is due to local (and possibly self-mediated) dispersal from each of the primary centres of establishment. The 2017 distribution of D. ramosus on the Danish islands (Fig. 2B) indicates a situation similar to that in Jutland 2010 (Fig. 2A): the species is present, but not abundant enough to turn up at every one-hour observation event.The situation is a logical consequence of the fact that in 2010 the species was completely missing on most of the eastern islands (except in Copenhagen). In another seven years we can expect it to have become firmly established on these islands too. Dicranopalpus ramosus were estimated to have invaded northern Europe with a speed of 35-100+ km per year. Range expansion rates at such speeds make it hard to imagine that the expansion could take place without human assistance. Though some flying insects (e.g. the harlequin ladybird beet- le, Harmonia axyridis ) may have spread with higher speeds (Brown et al. 2008, Hemptinne et al. 2012), most invasive winged insects have expanded with speeds in the same range as D. ramosus or slower (Hemptinne et al. 2012). Indeed, the spread of the harlequin beetle in Denmark has been consi- derably slower than that of D. ramosus (Steenberg & Harding 2009). Human vehicular transportation allows for dispersal by very long jumps also in species incapable of self-mediated long-distance dispersal. It even seems that the speed of ex- pansion has increased as the species moved north. This may be due to increased truck traffic between countries of the Eu- ropean Union during the last decennia. Building of bridges between Funen and Zealand (1998) and between Zealand and Sweden (2000) may have reduced the tendency of the sea belts between these islands (Storebælt and Øresund) to function as dispersal barriers by facilitating terrestrial traffic. Acknowledgements I am indebted to Axel Schönhofer and Hay Wijnhoven for valuable comments on the manuscript. References Arachnologische Gesellschaft 2018 Atlas of the European arachnids. – Internet: http://atlas.arages.de (1.III.2018) Artportalen 2018 Artportalen. Rapportsystem för växter, djur och svampar. – Internet: http://www.artportalen.se (1.III.2018) Brown PMJ, Roy HE, Rothery P, Roy DB, Ware RL & Majerus MEN 2008 Harmonia axyridis in Great Britain: analysis of the spread and distribution of a non-native coccinellid. – BioControl 53: 55-67 – doi: 10.1007/s10526-007-9124-y Enghoff H 1988 Operation Opilio 1987 – en undersøgelse af mejere på mure, stakitter o.l. steder i Danmark. – Entomologiske Med- delelser 56: 65-72